I'm a bit surprised to find that it took this long for an article about Frank Ramsey to appear in the SEP. Then again, Ramsey is an inherently difficult philosopher. But let me offer a very light (and incomplete, and in many ways wrong) version of Ramsey to give you an idea of his thinking. Suppose (Ramsey might say) a statement about probability isn't a statement about the world, but rather, a statement about you. So, for example, saying there's a 60% chance of something means that you would bet 60 to win 40. Does it add anything to say "and oh yeah, there's a thing, 'probability', actually in the world?" Well, no. So what does that tell us about mathematical facts and the nature of 'truth' in general? We call this the "subjectivist interpretation", and it's an important insight into the nature of mathematics and language.